1. Field of the Invention
The present invention relates to a system and a method for synchronization and transmission of information in a distributed measurement and control system. In particular, it may be used for real time transmission of information between physically connected nodes of an optical fiber network.
2. Description of the Related Art
The IEEE1588 standard enables synchronization of nodes which are distanced from each other up to 100 m, which corresponds to the maximum allowed length of an Ethernet link. Synchronization frames are transmitted between a master and a slave device, and the slave device should compute data necessary to adjust its own clock system.
The IEEE1588 standard requires at least four Ethernet frames to be transmitted between the devices in order to perform a single synchronization operation. These frames are used for synchronization only and do not carry any further useful information. The synchronization procedure is repeated every 1 ms. In order to transmit data, separate Ethernet frames must be used. Therefore, synchronization and data transmission according to IEEE1588 cannot be performed simultaneously.
Another transmission system is known from a publication by L. Sliwczynski, P. Krehlik, L. Buczek and M. Lipinski entitled “Active Propagation Delay Stabilization for Fiber-Optic Frequency Distribution Using Controlled Electronic Delay Lines” (IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 60, NO. 4, APRIL 2011). In that system, time markers are transmitted cyclically from a master node to a slave node, and the slave node returns the markers immediately to the master node. Due to the fact that the time of signal propagation is the same in both directions, the master node computes a time adjustment, which it then implements in its clock circuits in such a way that the time markers are sent to the slave with such time advance that the time markers in both devices appear at the same time point specified on a common time scale. The signals transmitted between the master node and the slave node in both directions do not carry any data. Such system enables precise synchronization of remote nodes, but does not enable simultaneous data transmission.
Switches used in transmission systems, such as switches used in Ethernet networks, transmit information frames received at their input to a specified output, depending on the address included in the frame header.
Isochronous real time (IRT) switches, such as the SCALANCE X204IRT switch, store received information in registers and transmit it to defined outputs at times specified by their internal clock.
There are also known hubs, which can be used as switches, and which send information received on all inputs to all outputs without any analysis of the information and without allocating those transmissions in time, which results in collisions.